Aircraft electronic landing responser system using scanning pencil beam ground antenna

ABSTRACT

An electronic approach and landing aid system involving the use of an airborne interrogator and a ground station having a rapid elevation frequency scan and a somewhat slower azimuth phase scan of a planar array. Each interrogating pulse produces a chirp pulse at the ground station and each chirp pulse produces one complete elevation scan cycle of the pencil beam formed by the planar array. Elevation angle data is air derived from the frequency of the pencil beam energy as it passes across the aircraft, and range is determined in proportion to ground chirp time between interrogating pulses and corresponding replies. Azimuth position of the ground antenna pencil beam is separately supplied through an independent modulation which is correspondingly decoded in the air.

United States Patent Sanders [54] AIRCRAFT ELECTRONIC LANDING 3,302,2011/1967 Vladimir et al. ..343/106 R RESPONSER SYSTEM USING 3,327,3096/1967 Shulman et al..... .343/16 R UX 3,349,399 10/1967 Bohm ..343/l06R ANTENNA Primary Examiner-T. H. Tubbesing 72 In t Lon L s d Pal V d P IAttorney-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. 1 van or Cam anOS er es enmsu a Hemminger, Charles L. Johnson, Jr. and Thomas E.Kristofferson [73] Assignee: International Telephone and TelegraphCorporation, New York, NY. [57] ABSTRACT [22] Filed: Mar. 30, 1970 Anelectronic approach and landing aid system involving the use of anairborne interrogator and a ground station having a [21] Appl' 23757rapid elevation frequency scan and a somewhat slower azimuth phase scanof a planar array. Each interrogating pulse [521 US. Cl. ..343/6.5 R,343/5 LS, 343111 R, Produces a chirp Pulse at the ground station andeach chirp 343/16 R, 343/ 17.2 R, 343/106 R pulse produces one completeelevation scan cycle of the pen- 5 1 [111.(11 ..Gls 9/56 cil beam formedby the Planar Elevation angle data is 58] Field of Search .343/ LS, 6.5R, 1 1 R, 16 R, air derived from the frequency of the Pencil beam energyas it 343/172 R 106 R passes across the aircraft, and range isdetermined in proportion to ground chirp time between interrogatingpulses and [56] References Cited corresponding replies. Azimuth positionof the ground antenna pencil beam is separately supplied through anindependent UNITED STATES PATENTS modulation which is correspondinglydecoded in the air. 3,266,038 8/1966 Milne et al. ..343/1 1 R X Claims,1 Drawing Figure BE A/ (/7 6/ WTERROGATE m/rERRoe/WE 12 5 6 *6TRANSM/UZR UYNCHROIV/ZER a 5 I RECE/VER g 2/ 52- CK/M/AHRJR CHIRPAMPLITUDE o /5 TRANSM/TIZ'R MODULATOR RANGE 33 f 1 Rem/5mm R A /00Ml-l=nCKR CIRCUIT 2? 362 1/ 11 20 22 FHA-56 5 ga 2-0x 2-OP/VC/L 56AM I 3 A Z/0OMH=-20 86AM flA/GLE /O/MHz/BAMW/DTH 0575c 70R TIME GEM /o MHz/U556 /o55 2@ Ave. FREQ L AMPLITUDE BEAM I AS Dew/000mm? SAMPLER CHART 6 5 /4Patented March 7, 1972 INVENTOR. 10m A. SANDERS AIRCRAFT ELECTRONICLANDING RESPONSER SYSTEM USING SCANNING PENCIL BEAM GROUND ANTENNABACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates broadly to air navigation systems and more particularly toinstrument landing systems for obtaining air derived range and angleinformation.

2. Description of the Prior Art In the prior art there have been manydevelopments to provide navigational guidance to aircraft. Such systemshave been produced for long range or en route navigation (LORAN,OMNIRANGE, RADAR in various forms, etc.), as well as for the relativelymore critical problem of providing navigational assistance under lowvisibility instrument conditions at the time of landing. The urgency ofthe landing situation has long been recognized as a problem of very highpriority since it critically affects safety and continuity of operationin civil and military aviation.

Existing systems for landing include lLS, GCA, automatic GCA, andvarious other active ground and airborne systems. A comprehensivereference for reader background which describes most of the better knowncurrently used prior art navigational aids for aviation is the textbookElectronic Avigation Engineering by P. C. Sandretto, published in 1958by International Telephone and Telegraph Corporation, New York, NY.

One embodiment of the so called GCA talk down system is described in US.Pat. No. 2,975,413. Its automatic trackwhile-scan form is typicallydescribed in US. Pat. No. 2,980,902.

Various ways have been proposed for using radar for specialized landingassist, however, the relative complication and operating personnelrequirements can be seriously disadvantageous in the case of the remoteor newly established landing area for aircraft of the VTOL classes.Landing aids for VTOL aircraft in all-weather conditions are of specialinterest in certain military situations and are attracting increasinginterest in other situations.

For the type of device with which the present invention is concerned, afew specific applications are as follows:

a. A landing aid for VTOL fighter-bomber aircraft for use at remotedispersal air fields or parking pads.

b. A landing aid for soft helicopter operations in remote areaspreviously accessible only by parachute.

c. A landing aid for civil helicopter and VTOL airways operations foruse in urban areas.

d. A landing strip aid for forward airfields or cargo delivery airstrips.

e. A landing aid which augments an [L8 system or a GCA system at larger,well improved airports.

The above areas of need represent existing situations where there is ahigh premium on all-weather operations and a large number of potentialuser aircraft. None of the above requirements can be effectivelysatisfied by the existing lLS or GCA type systems.

The disadvantages of the said prior art systems concern data rates,angles of coverage, power economy, ability to operate at remote groundstations unattended, and lack of ground station radiation security.

SUMMARY In view of the applications for devices of a class of thepresent invention, and in consideration of the disadvantages of theprior art devices, it may be said that the general object of the presentinvention was the implementation of a landing aid system providing forair-derived angle and range information, which is especially adapted tothe VTOL type of aircraft.

The present invention provides for relatively simple airborne equipmentworking in cooperation with a ground station in the vicinity of alanding area. The airborne equipment contains both transmitting andreceiving functions operating over the same wide band antenna andcirculator feed systems for simultaneous transmission and reception. inthe transmitting mode, the airborne equipment transmits a relativelyshort interrogating pulse at a relatively low repetition frequency(approximately 250 Hz). The radio frequencies involved in transmittingthese interrogation pulses is removed by at least a nominal separationfrom the band of frequencies to be retransmitted from the groundstation.

At the said ground station, the heart of the system comprises atwo-dimensional scanning antenna which forms a relatively sharp pencilbeam. The said pencil beam is positioned in a first angular coordinate(elevation typically) by means of frequency scanning, and in a secondangular coordinate of scan (typically azimuth) by an independent phasescan. in response to each interrogating pulse, a receiver-transmittercombination at the ground station generates a chirp (frequencymodulated) pulse of duration and frequency deviation such as to causeone complete cycle of elevation scan in the said ground antenna for eachapplied chirp pulse. The azimuth scan control means are preferablyoperated continuously and whenever the said ground antenna is energizedin response to the aforesaid airborne interrogation pulses, the saidpencil beam is formed at an azimuth corresponding to the instantaneouscondition of this azimuth control. A typical frequency phase planararray and scanning system suitable for the ground is described in US.Pat. No. 3,398,365.

From the foregoing, it will be understood that the passage of the saidpencil beam over the interrogating aircraft will be delayed somefraction of the time of an azimuth scan. This delay, however, is notsignificant in terms of aircraft movement, since the azimuth scan wouldtypically be on the order of five cycles per second.

Upon passage of the pencil beam over the interrogating aircraft, theinstantaneous chirp frequency or rather band of frequenciescorresponding to the elevation width of the pencil beam would bereceived and detected in a discriminator circuit having an output analogversus frequency characteristics which is the same function as that ofthe elevation beam pointing position versus frequency of excitation atthe ground antenna.

Range is air derived by tracking the reply chirp pulses. The delay ofeach chirp pulse following a corresponding interrogation pulse isproportional to two-way radiation time and therefore to range, as iswell understood. In the present system, a correction of the range analoggenerated by such a range tracker is necessary, however, since thebeginning of the chirp pulse always corresponds to one extreme of theelevation scan of the ground antenna. Typically, the chirp pulse is 10microseconds in duration and accordingly, without correction, the rangeanalog could be in error by as much as 10 microseconds which wouldproduce an error of an appreciable fraction of a mile. Since the saiderror will be seen to be a function of elevation angle, the opportunityis taken to subtract a suitably scaled elevation analog signal from theuncorrected range analog. The result is a greatly improved range analogaccuracy. If the ground antenna elevation scan beginning point is at thelowest elevation angle, the importance of the range analog correctionjust described will be seen to be diminished as the aircraft approachesits landing area and range and elevation angle both approach zero. Under'these circumstances the passage of a pencil beam over an approachingaircraft occurs early in the chirp pulse (low elevation angle).

At the ground station, an azimuth angle tone generator is coupled to theazimuth scan control means in a manner so as to vary the amplitude of aconstant signal which is then superimposed through an amplitudemodulator on the chirp pulses to be transmitted from the ground station.This process will be described in more detail as this specificationproceeds. The use of a low frequency audio tone in the 50 to cycleregion for the basic signal variable amplitude signal in this azimuthscan angle analog scheme will be described. it will be readily realized,however, that other modulation schemes could be employed in thisconnection and, in fact. the said azimuth angle analog could betransmitted over the separate antenna and at the separate frequenciesover which the airborne interrogations are received.

The azimuth data transmission feature of the overall combination is lessfundamental to the novel concepts than is the elevation angledetermination structure.

The decoding of the azimuth angle analog information in the airborneequipment simply involves the use of an amplitude demodulator whichincludes low pass filtering to eliminate any frequency componentsresulting from the system repetition frequency and extraneous higherfrequencies. The output of such a demodulator contains the basic azimuthangle information. however, its quality and utility are improved if theoutput thereof is gated by a signal having an on-time equal to the timeof the pencil beam passage over the aircraft. Such a gating signal isderived by box-car detection of the airborne receiver output. Thiscombination may be referred to as angle gating of the azimuth amplitudedemodulator output. Thereafter an average frequency measuring circuit,which is itself a frequency discriminator. produces an azimuth angleanalog.

Since radiation security from the ground station requires thatnonuseable transmission be minimized, the said beam detector gate isused to control an inhibiting timer which operates to disable the sourceof airborne interrogation pulses at all times other than during azimuthalignment of the pencil beam of the ground antenna with theinterrogating aircraft. An inhibiting timer effectively learns when toexpect the next azimuth alignment of the pencil beam and enablesoperation of the airborne interrogating circuits at those times.

The details of circuits and elements required to implement the system ofthe present invention will be described as this specification proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE is a structural andfunctional block diagram illustrating the air and ground elements of acomplete rystem according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the FIGURE acomplete structural and functional block diagram of the system of thepresent invention is shown. The antenna 1 depicted as a hom-typeradiator, and all components to the right of it on the FIGURE, are to beunderstood to comprise the airborne components of the system. On theother hand, antenna 2 and all of the components to the left of it are tobe understood to be located at the ground station adjacent to thelanding area in connection with which the present system is intended tofunction.

The system to be described is essentially a two-way radiating systeminvolving interrogation from the air to a ground station. The saidinterrogation pulses from the air are answered on a one for one basis bya relatively long chirped pulse t frequency modulated) groundtransmission. Of the three antennas envisioned in the present system,the airborne antenna 1 and the ground antenna 3 are not essentiallydirective, whereas the antenna 2 is a planar array capable of pointingcontrol of a pencil beam in two coordinates. An antenna and scanningsystem entirely suitable for this application is described in U.S. Pat.No. 3,438,035. That antenna is a planar array adapted for simultaneousfrequency and phase scanning. The frequency scan is capable of changingthe beam pointing angle very rapidly since it is essentiallyinertialess. The phase scan accomplished in the orthogonal plane is, onthe other hand, accomplished at a lower rate. The mechanically actuatedform of phase scan illustrated in the said U.S. Pat. No. 3,438,035 issufficiently fast for providing the four or five cycle per secondazimuth scan contemplated in the system of the present invention. Thefaster acting electronically controlled phase shifting embodiment shownin the same reference may be used but is not required.

The present system contains the required instrumentation for the airderivation of azimuth and elevation angular data as well as range.Azimuth and elevation angle data is derived with respect to the groundstation, as is usually the case with electronic landing aids. In thedescription to follow. first and second angular coordinates have beenreferred to for the sake of generality. Normally the said first andsecond angular coordinates would refer to the elevation angle and theazimuth angle (bearing) respectively.

in order to meet operational requirements for VTOL landing, the groundstation antenna 2 would typically be designed to frequency scan in theelevation plane over a total 20 sector. A MHz frequency change or chirpof the transmitted pulse from 2 could be expected to accomplish theaforementioned 20 range of beam pointing angles, in a system operatingbasically at a transmitted frequency somewhere in the region of 5,000 tol5,000 MHz. The duration of the transmitted chirp pulse from the groundantenna 2 is 10 microseconds in one practical embodiment, that is, the100 MHz chirp or frequency deviation occurs within a 10 microsecondpulse.

The planar array 2 is substantially symmetrical insofar as number ofradiators is concerned and is designed to produce a pencil beam ofapproximately 2 at half power points, in each of the two orthogonalplanes of scan.

Although it has been indicated that antennas l and 3 are substantiallynondirectional, they are illustrated as horn antennas which obviouslyhave some directivity. Those directivity characteristics are notnecessary to the functioning of the present system, however, somedirectivity is desirable for purposes of radiofrequency power economyand to minimize interference problems. It is a requirement that groundantenna 3 provide a sufficiently broad radiation pattern to cover thesaid 20 elevation scan sector through which the planar array 2functions, and in addition an arbitrary azimuth sector (up to 45typically). Antenna 1, which radiates the interrogation pulses towardthe said ground station, needs to have a sufficiently broad radiationpattern to ensure that the interrogation pulses are received at 3 forall aircraft positions within the approach sector, allowing also fornominal aircraft attitude deviations.

The airborne synchronizer 6 and transmitter 5 cooperate to energize theantenna 1 with the relatively short duration interrogation pulses(typically 1 or 2 microseconds in duration) at a relatively low pulserepetition frequency of 250 Hz. A circulator 4 is to be understood to bea straightforward prior art three-part device of the ferrite or similartype. It functions to pass the said interrogation pulses via lead 29 toantenna 1.

in passing pulse energy from the transmitter 5 to the antenna 1 thecirculator 4 diverts substantially no energy into the lead 30.Conversely, when receiving chirp pulse energy from the ground stationthe circulator 4 operates to pass the said received energy to lead 30and thence to the wide band receiver 8 without substantial diversioninto the lead 29.

Before proceeding with functional descriptions it is desirable todiscuss the details of the ground station equipment more fully. It willbe assumed that the mechanical phase scan version in the aforementionedU.S. Pat. No. 3,438,035 is employed in connection with the groundstation antenna 2 and its associated hardware. The phase scan drive 36is mechanically iinked by a representation 37 and it is presumed thatthe mechanical action which produces the phase scan operatescontinuously whenever the ground station is in a condition receptive tointerrogation from the airborne equipment. This continuous operation ofthe mechanical phase scan structure of the antenna 2 does not mean thatany transmission is being radiated from the antenna 2. The continuousoperation of these mechanical components is a convenience andsimplification rather than a functional necessity.

An azimuth angle tone generator 35 is linked by a connection 38 to thesaid mechanical drive 36. The tone generator 35 operates typically in aband from 50 to 100 Hz. and the frequency it produces and applies vialead 39 to the amplitude modulator 34 varies in manner directly relatedor analogous to the pencil beam radiation angle from the antenna 2 inthe azimuth or second beam angle coordinate. Analog data takeoffs ofthis particular type are well known in the prior art and theirinstrumentation is readily undertaken by the skilled practitioner inthis art.

Thus, it will be seen that a continuous azimuth-representing signal isprovided to the amplitude modulator 34 via lead 39 and this signal isavailable as an amplitude modulation to be superimposed on the signalfrom the chirp transmitter 33 whenever the latter is brought intooperation. Referring now to the synchronizer 6 which functions as atiming unit for the airborne components, an initiate signal applied at 7begins the cycle of interrogation and operation of the system. Thissignal at 7 is merely a switching or enabling function and may simply bethought of as an operator on-off control. The inhibiting time of 17which will be described in greater detail later is not operative at thetime of initiating the interrogation pulses.

The said interrogation pulses once received by the antenna 3 in thereceiver 31 at the ground station, are provided via lead 32 assynchronizing pulses to the chirp transmitter 33. The signal on 32 maybe thought of as a straightforward video pulse at the said interrogationPRF on the order of 250 Hz. The chirp transmitter 33 is to be understoodto contain the necessary frequency modulation circuits to generate theindividual l0 microsecond chirp transmitter output pulses at the samepulse rate repetition frequency as received from the aircraft. Thesechirp pulses are parametered to correspond with the elevation angle vs.frequency characteristic of the antenna 2. That is, in the microsecondduration of each chirp pulse the frequency modulation excursion is 100MHz, which produces a 20 beam pointing angle change (scan) in theelevation or first angular coordinate. The tone modulation correspondingto the azimuth beam pointing position being continually present on lead39, the amplitude modulator 34 superimposes a corresponding azimuthindicating signal on the chirp pulse series from 33. In this way thetransmitted information from the antenna 2 provides the basicinformation from which both elevation and azimuth angles can be airderived.

It will be realized, in view of the relatively short duration of thechirp pulses, that the dwell or passover time of the pencil beam fromantenna 2 at any particular aircraft angle is only l microsecond. Thefact follows from the total chirp pulse duration of 10 microseconds andthe ratio of the elevation pencil beam width to the total elevationscan.

Once enabled, the interrogation pulses are continuously transmitted andchirp pulse replies are transmitted immediately in response to theinterrogations. However, these are not received at the antenna 1 unlessand until the continuously scanning azimuth beam pointing angle arrivesat the angle corresponding to the interrogating aircraft. Once thiscondition is satisfied however, the said I microsecond bursts of chirpedenergy (containing a 10 MHz segment of the total chirp) are received.Since the azimuth scan is relatively slow and the azimuth beam width ofthe antenna 2 is on the order of 2", there will be several elevationchirp cycles during the time that the pencil beam can illuminate theantenna 1 during each azimuth scan cycle.

The pulses received at 1, after passing through the circulator 4, travelvia lead 30 through the wide band receiver 8 and thence to thediscriminator 9. This discriminator 9 is calibrated to provide an analogsignal output at 21 which is the same function of frequency at 23 as theelevation beam angle versus frequency function of the antenna 2. Thediscriminator 9 is to be understood to contain the necessary smoothingand integrating circuitry so that the said elevation angle analog signalat 21 is based on the average chirp frequency within the l microsecondreceived pulses as aforesaid. This analog signal on 21 is provided to anelevation angle analog output terminal 28 and also to a range refinementcircuit which will be described in more detail in connection with therange determining circuits.

The envelope of the signal at 23 will be seen to resemble a typicalradar echo pulse. As such it is readily handled by time delayedmeasuring circuits for the determination of range. It will be noted thata synchronizing pulse on lead 20 from the synchronizer 6 is supplied tothe range tracker l0 marking the beginning of each interrogation pulse.So long as there are no appreciable time delays introduced in the groundstation the range from the aircraft to the. said ground station isdirectly proportional to the round trip radiation transmission time.That is, the time from the beginning of a selected interrogation pulseuntil the corresponding chirp pulse is received at the aircraft. Therange tracker 10 as such is readily constructed by one skilled in thesearts. A relatively elaborate circuit such as shown for range tracking inU.S. Pat. No. 2,795,781 could provide this function, or a simpletechnique such as a ramp reference voltage generator with sample andhold circuits could provide the function of block 10. In the latterapproach, the synchronizing pulse supplied by a lead 20 would be used toinitiate a linear sawtooth or ramp voltage having a peak voltage andduration corresponding to the maximum anticipated range of the system.The signal input from 23 would then be used as a sampling pulse wherebythe said sawtooth amplitude is sampled during the time of occurrence ofthe signal at 23. Suitable smoothing and holding circuits can provide avoltage analog of range at 22.

Upon careful consideration of the aforementioned range determinationfunction, it will be realized that correction of the analog signal on 22is necessary because an additional delay is added to the round triptransmission time proportional to the elevation angle. This error can beas much as the entire chirp pulse, i.e., l0 microseconds or asubstantial fraction of a mile. In view of this effect, the output ofthe elevation angle discriminator 9 at 21 is supplied to the rangerefinement circuit 15. This circuit is simply an appropriately scaledalgebraically additive mixer in which the signals at 22 are reduced by afactor proportional to the said range analog on 21. Stated otherwise, afactor proportional to the elevation angle analog signal is subtractedfrom the range analog in order to produce a corrected range analogoutput at 27.

It will be noted that the output of the wide band receiver at 23 is alsoapplied to a beam detector 11 and an amplitude demodulator 12 inparallel. The said beam detector 11 is simply a circuit for generating asquare wave which is representative of the angular envelope of the beamdwell or passby time during elevation scan. The signal at 23 is firstsubjected to a threshold circuit in 11 in order to eliminate much of thenoise, sidelobes and other relatively low extraneous amplitude signals.Accordingly, the output from the beam detector 11 at 16 is a gate on theorder of the l microsecond in duration appearing at the repetitionfrequency of the system. Using this signal 16 as a gate, a beam sampleror electronic switch 13 eliminates from the output 24 of the amplitudedemodulator 12 all signals occurring at times outside the said gate.Thus the signal on lead 25 may be thought of as being angle gated andcontaining essentially the same band of chirp frequencies as wereapplied to the discriminator 9. An average frequency measurement circuit14 (which is in effect another discriminator operating in a much lowerfrequency range) then provides an output 26 which is an analog of theazimuth angular position of the aircraft with respect to the groundstation.

From the point of view of radiation security, particularly in hostileenvironments, it is desirable that the ground station not transmit for alonger time than necessary or over angles other than those correspondingto an aircraft on approach. Since there is essentially random access tothe ground station, the azimuth pointing angle of the antenna 2 may beany angle within its full azimuth scan range at the time of reception ofthe first interrogation pulse from the air. Since chirp scan replies arenot received by the aircraft until such time as the azimuth beampointing angle reaches the bearing of the corresponding aircraft, someextraneous ground transmission is inevitable. However, since the scanperiod of the ground antenna 2 in azimuth is known in the aircraft andis constant, the opportunity of timing the synchronizer to interrogateonly during those times when a response can be received in the aircraftsuggests itself. Thus a simple timing function applied to thesynchronizer 6 to keep it from generating interrogating synchronizingpulses except during the beam detector gate on l6 is possible. The onlyadditional requirement is for phase locking the inhibiting timer 17 tothe said gate. Over several cycles of azimuth scan, at the most, theinhibiting timer 17 can leam the correct phase for its operation. Aself-timing gate which generates a blanking or inhibiting pulse having aduration equal to the time between azimuth scan passes over an aircraftcan automatically enable the synchronizer 6 via a signal on 18 inanticipation of each succeeding azimuth signal azimuth beam pass. Thecircuit elements required for instrumentation of the block 17 will beevident to those skilled in this art from the foregoing requirements andfunctional description.

it will be realized that the chirp pulse time as a percentage of theinterpulse period of the interrogation pulses is quite small, i.e., theairborne equipment operates on a low duty cycle. Thus airborne powereconomy is relatively good; and the combination of the low duty cycleoperation required of the ground station and in view of theexclusiveness of the transmissions of the ground station on an azimuthangular basis, it will be realized that a number of aircraft ondifferent approach paths in azimuth and in elevation may besimultaneously accommodated by the present equipment.

Obviously, since the individual circuit block functions are ofthemselves relatively simple. a number of alternative instru-.mentations are possible. Also, certain system modifications within thescope of the present invention are possible. It will be evident that theazimuth angle analog signal information transmitted from the groundcould also be transmitted over the antenna 3 by converting the amplitudemodulator 34 into a small amplitude modulator-transmitter, feeding itsoutput into a circulator inserted in the path between antenna 3 andreceiver 31. Normally the frequency band used for transmissions betweenantennas l and 3 would be separated from the range of useful frequenciestransmitted by the antenna 2, and accordingly such an expedient wouldnot introduce any additional interference complications.

it will also be realized that pulse repetition frequencies, chirp pulseduration and frequency deviation factors are typical and exemplary only,as is the type of modulation employed to transmit the azimuth angledata.

it is not intended that the scope of the present invention should belimited by the description or the drawing, these being typical andillustrative only.

What is claimed is:

.l. An air navigation system particularly adapted for instrument landingof VTOL aircraft, comprising the combination of:

a ground station located adjacent to a landing area;

airborne transmitting means including a synchronizer for generating andradiating a series of interrogation pulses toward said ground station,at a relatively low pulse repetition frequency;

means for receiving said interrogation pulses at said ground station andfor generating a chirp transmitter pulse for each of said interrogationpulses received;

ground antenna means for radiating said chirp pulses at an angle in afirst angular coordinate which corresponds continuously with theinstantaneous chirp frequency at any time during said chirp pulse,thereby to produce a chirp scanned beam in said first angular coordinatefrom said ground antenna;

airborne receiving means carried by said aircraft for receiving energyfrom said ground station as said scanning beam passes over saidaircraft;

discriminator means responsive to the output of said airborne receivingmeans, said discriminator being calibrated to provide an output inaccordance with the same function of frequency as the angle ofradiation-versus-frequency function of said ground antenna, whereby saiddiscrimination output is the analog of the beam pointing angle of saidground antenna in said first angular coordinate.

2. The invention set forth in claim 1 including range tracking meansresponsive to the output of said receiving means and said synchronizerfor producing a range approximation signal as a function of the timebetween an interrogation pulse and reception of said energy when saidground antenna beam crosses said aircraft in said first angularcoordinate scan.

3. The invention set forth in claim 2 including a range refinementcircuit responsive to said range approximation signal and saiddiscriminator output for subtracting a factor from said rangeapproximation signal proportional to the delay of the input signal tosaid range tracker from the leading edge of said chirp pulse, thereby toproduce a refined range signal having a value substantially proportionalto the signal transit time between said aircraft and said groundstation.

4. The invention set forth in claim 3, further defined in that saidground antenna means includes at least a linear array for forming a beamnarrow in the plane of said first angular coordinate, said arrayincluding a plurality of elements fed from a transmission linepresenting a predetermined extended path between adjacent elements ofsaid array to produce beam pointing in said plane of said first angularcoordinate which is a predetermined function of frequency.

5. The invention set forth in claim 3 in which said ground antenna meanscomprises a planar array for forming a pencil beam; additional scanningmeans are included for simultaneously and independently scanning saidpencil beam in a second angular coordinate at an angle with respect tosaid first angular coordinate; means operatively associated with saidadditional scanning means and said means for generating said transmitterchirp pulse, for introducing a separate and independent modulation onsaid chirp pulses to represent the position of said pencil beam withinsaid second angular coordinate; and additional airborne angle detectionmeans for decoding said separate modulation thereby to generate a signalrepresentative of the aircraft position in said second angularcoordinate.

6. The invention set forth in claim 5 in which said separate modulationis defined as being amplitude modulation, and said additional airborneangle detection means includes an amplitude demodulator having an outputrepresentative of said aircraft position in said second angularcoordinate.

7. The invention defined in claim 5 in which said planar array consistsof a plurality of columns of radiators, said columns extending withinthe plane of said chirp scanned beam and said columns beingsimultaneously supplied said chirp pulses to produce said beam scan inthe plane of said first angular coordinate; and said additional scanningmeans includes means for varying the phase of excitation of said columnsdifferentially and progressively throughout all of said columns in saidplanar array.

.8. The invention defined in claim 7 in which said first scanningcoordinate is defined as that of elevation angle measurement from saidground station, and said second angular coordinate is defined as that ofazimuth angle measurement from said ground station.

9. The invention set forth in claim 7 including means for generating anangle gate during the time said pencil beam scans over said aircraft andinterrogation limiting means for applying said angle gate to saidairborne transmitting means to enable the generation of saidinterrogation pulses substantially only during said angle gate, saidinterrogation limiting means including timing means operative after aninitial period of interrogation equal to at least one scan cycle of saidadditional scanning means and adapted to inhibit transmission of saidinterrogating pulses substantially during the time said pencil beam isnot pointing toward said aircraft.

average frequency measuring circuit responsive to the output of saidsampling circuit is also included for producing said output signal whichis representative of said aircraft position in said second angularcoordinate.

1. An air navigation system particularly adapted for instrument landingof VTOL aircraft, comprising the combination of: a ground stationlocated adjacent to a landing area; airborne transmitting meansincluding a synchronizer for generating and radiating a series ofinterrogation pulses toward said ground station, at a relatively lowpulse repetition frequency; means for receiving said interrogationpulses at said ground station and for generating a chirp transmitterpulse for each of said interrogation pulses received; ground antennameans for radiating said chirp pulses at an angle in a first angularcoordinate which corresponds continuously with the instantaneous chirpfrequency at any time during said chirp pulse, thereby to produce achirp scanned beam in said first angular coordinate from said groundantenna; airborne receiving means carried by said aircraft for receivingenergy from said ground station as said scanning beam passes over saidaircraft; discriminator means responsive to the output of said airbornereceiving means, said discriminator being calibrated to provide anoutput in accordance with the same function of frequency as the angle ofradiation-versus-frequency function of said ground antenna, whereby saiddiscrimination output is the analog of the beam pointing angle of saidground antenna in said first angular coordinate.
 2. The invention setforth in claim 1 including range tracking means responsive to the outputof said receiving means and said synchronizer for producing a rangeapproximation signal as a function of the time between an interrogationpulse and reception of said energy when said ground antenna beam crossessaid aircraft in said first angular coordinate scan.
 3. The inventionset forth in claim 2 including a range refinement circuit responsive tosaid range approximation signal and said discriminator output forsubtracting a factor from said range approximation signal proportionalto the delay of the input signal to said range tracker from the leadingedge of said chirp pulse, thereby to produce a refined range signalhaving a value substantially proportional to the signal transit timebetween said aircraft and said ground station.
 4. The invention setforth in claim 3, further defined in that said ground antenna meansincludes at least a linear array for forming a beam narrow in the planeof said First angular coordinate, said array including a plurality ofelements fed from a transmission line presenting a predeterminedextended path between adjacent elements of said array to produce beampointing in said plane of said first angular coordinate which is apredetermined function of frequency.
 5. The invention set forth in claim3 in which said ground antenna means comprises a planar array forforming a pencil beam; additional scanning means are included forsimultaneously and independently scanning said pencil beam in a secondangular coordinate at an angle with respect to said first angularcoordinate; means operatively associated with said additional scanningmeans and said means for generating said transmitter chirp pulse, forintroducing a separate and independent modulation on said chirp pulsesto represent the position of said pencil beam within said second angularcoordinate; and additional airborne angle detection means for decodingsaid separate modulation thereby to generate a signal representative ofthe aircraft position in said second angular coordinate.
 6. Theinvention set forth in claim 5 in which said separate modulation isdefined as being amplitude modulation, and said additional airborneangle detection means includes an amplitude demodulator having an outputrepresentative of said aircraft position in said second angularcoordinate.
 7. The invention defined in claim 5 in which said planararray consists of a plurality of columns of radiators, said columnsextending within the plane of said chirp scanned beam and said columnsbeing simultaneously supplied said chirp pulses to produce said beamscan in the plane of said first angular coordinate; and said additionalscanning means includes means for varying the phase of excitation ofsaid columns differentially and progressively throughout all of saidcolumns in said planar array.
 8. The invention defined in claim 7 inwhich said first scanning coordinate is defined as that of elevationangle measurement from said ground station, and said second angularcoordinate is defined as that of azimuth angle measurement from saidground station.
 9. The invention set forth in claim 7 including meansfor generating an angle gate during the time said pencil beam scans oversaid aircraft and interrogation limiting means for applying said anglegate to said airborne transmitting means to enable the generation ofsaid interrogation pulses substantially only during said angle gate,said interrogation limiting means including timing means operative afteran initial period of interrogation equal to at least one scan cycle ofsaid additional scanning means and adapted to inhibit transmission ofsaid interrogating pulses substantially during the time said pencil beamis not pointing toward said aircraft.
 10. The invention set forth inclaim 6 further defined in that said additional airborne angle detectionmeans includes a sampling circuit responsive to the output of saidamplitude demodulator; means are included for enabling said samplingcircuit during passage of said beam at said aircraft; and an averagefrequency measuring circuit responsive to the output of said samplingcircuit is also included for producing said output signal which isrepresentative of said aircraft position in said second angularcoordinate.